1. Field of the Invention
The present invention relates to a method of and an apparatus for controlling the steering operation of a motor vehicle having steerable front and rear road wheels, and more particularly to a method of and an apparatus for controlling the steering operation of a motor vehicle in which rear road wheels are steerable by an actuator such as an electric motor in response to the steering action of front road wheels.
2. Description of the Relevant Art
There are known motor vehicles in which both front and rear road wheels are steerable. In some of such motor vehicles, the direction and angle of steering movement of the rear road wheels with respect to the front road wheels are controlled dependent on the speed of travel of the motor vehicle. When steering the motor vehicle in a low vehicle speed range, the rear road wheels are steered in the opposite direction (opposite phase) to the front road wheels for allowing the motor vehicle to make a turn of a small radius. When steering the motor vehicle in a high vehicle speed range, the rear road wheels are steered in the same direction (in phase) as the front wheels for increased motor vehicle stability.
One known motor vehicle with steerable front and rear road wheels is disclosed in Japanese Laid-Open Patent Publication No. 60-67272. According the steering control apparatus disclosed in this publication, rear road wheels are actuated by a step motor the step angle of which is controlled according to the speed of steering action of the motor vehicle. When the steering speed is low, the step angle of the step motor is reduced to turn the rear road wheels in smaller increments for highly accurate steering control. When the steering speed is high, the step angle of the step motor is increased to turn the rear road wheels in greater increments for increased steering response.
The disclosed steering control apparatus, however, does not take into account the turning behavior of the motor vehicle at a high steering speed. The output torque of the step motor is constant, the angle of rotation of the step motor corresponds to the steering angle of the rear road wheels at a predetermined ratio, and the steering angular speed of the rear road wheels is determined solely dependent on the speed of rotation of the step motor.
The steering angle of the rear road wheels is only controlled so as to be simply equalized to a preset target steering angle. Other control factors, such as the coefficient of friction of a road, are not taken into consideration by the disclosed steering control apparatus. More specifically, the motor vehicle does not have turning characteristics in accord with the cornering forces produced on the rear road wheels while the motor vehicle is running at high speed and also with a large road resistance imposed by the road while the motor vehicle is running at low speed.